qemu_process: Enter QMP command mode when starting QEMU Process
    
    qemuProcessQMPStart starts a QEMU process and monitor connection that
    can be used by multiple functions possibly for multiple QMP commands.
    
    The QMP exchange to exit capabilities negotiation mode and enter command
    mode can only be performed once after the monitor connection is
    established.
    
    Move responsibility for entering QMP command mode into the
    qemuProcessQMP code so multiple functions can issue QMP commands in
    arbitrary orders.
    
    This also simplifies the functions using the connection provided by
    qemuProcessQMPStart to issue QMP commands.
    
    Test code now needs to call qemuMonitorSetCapabilities to send the
    message to switch to command mode because the test code does not use the
    qemuProcessQMP command that internally calls qemuMonitorSetCapabilities.
    
    Signed-off-by: Chris Venteicher <cventeic@redhat.com>
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
    Reviewed-by: Ján Tomko <jtomko@redhat.com>